1. Field of the Invention
This disclosure presents improvements of the traveling wave field effect transistor (TWFET) described in U.S. Pat. No. 5,627,389 and the U.S. continuation-in-part patent No. 5,861,644. These patents are hereby incorporated herein by reference. As the disclosure explains, the present invention allows the use of lower conductivity metals for the electrodes of the TWFET, and provides means of adjusting the high-frequency current-voltage characteristics of the TWFET for use in a TWFET amplifier or an oscillator circuit element.
2. Background Information
These new TWFET features, and the new TWFET design will be described after a brief review of the TWFET which was also presented previously in U.S. Pat. No. 5,627,389 and the continuation-in-part U.S. Pat. No. 5,861,644. This review will provide a basis for a description of the new TWFET design features and the new TWFET design.
The TWFET structure, as described in the U.S. patents referenced above, is a semiconductor structure based on a conventional metal-semiconductor field effect transistor (MESFET) with elongated gate, source, and drain electrodes. These electrodes are placed in parallel with each otherxe2x80x94aligned with the length axis of the electrodes. Also, as in the case of the conventional MESFET, the electrodes are located on the surface of a semiconductor and are connected by an active channel region of the semiconductor.
While the TWFET has structural similarities to the conventional MESFET, an important difference between these two types of transistors is the direction of signal propagation. The TWFET signal propagation direction is along the length-axis of the elongated gate, source and drain electrodes. This is perpendicular to the signal propagation direction of a conventional MESFET. The TWFET signal propagation direction helps to relate the TWFET structure to the conventional MESFET structure. Specifically, in a TWFET, a plane transverse to the direction of signal propagation defines a cross-section of this TWFET structure, and in this plane, the electrode placement and active channel region of the TWFET structure corresponds to that of a conventional MESFET. As described in U.S. Pat. Nos. 5,627,389 and 5,861,644, the MESFET activity and interelectrode coupling of the TWFET, combine with the coupling length and electrode series resistance values to determine the high frequency current-voltage characteristics of the TWFET transistor. This same TWFET analysis provides the basis for discussion of the present invention, and is briefly reviewed as follows:
The analysis of the TWFET operation is based on its correspondence to a coupled transmission line structure. In the TWFET, the elongated gate, source, and drain electrodes act as two pairs of coupled transmission lines, in which the length of these electrodes provides a coupling length for the structure. The coupling of these electrode-transmission lines takes place with the high frequency AC activity of the cross-section FET of the TWFET structure. This activity is included in the analysis via the coupling matrices of the transmission line differential equations for the TWFET structure:                     ⅆ                  ⅆ          z                    ⁢      i        =          -      Yz                          ⅆ                  ⅆ          z                    ⁢      v        =          -      Zi      
Here i and v are two-element current and voltage vectors of the two coupled transmission lines in the TWFETxe2x80x94the gate-source pair of electrodes and the drain-source pair of electrodes. The admittance coupling matrix, Y, describes the AC current flow in the plane transverse to the direction of signal propagation. It is identical to the admittance matrix which describes the AC current-voltage relationship of the conventional FET which forms the cross-section of the TWFET. The impedance coupling matrix, Z, is defined as the sum of the diagonal series resistance matrix, R and the product of jxcfx89 with L, the inductive coupling matrix. As described in U.S. Pat. Nos. 5,627,389 and 5,861,644, the diagonal series resistance contains the series resistance of the gate-source and drain-source electrode pairs that form the two transmission lines of the coupled structure. Also, as discussed in these earlier patents, a TEM mode approximation of the propagating signal is used in this analysis. This approximation relates the inductive coupling matrix, L, to the AC charge matrix, K. This AC charge matrix is obtained from calculations of the AC charge which appears on the gate and drain electrodes with applied AC bias conditions.
In addition to the calculation of the coupling matrices, another component of the analysis is that it is applied to the case of a dual-gate TWFET, such as the TWFET example of FIG. 3 of U.S. Pat. No. 5,627,389. In this structure, the incoming signal is divided into the input side of two identical TWFETs which share a common drain electrode. This structure is analyzed by doubling the effective contact area for all of the electrodes. This has the result of doubling the matrix element values of the admittance coupling matrix Y and of the charge coupling matrix K. In addition, the doubled contact area reduces the series resistance value by a factor of two, as described in the patents reference above.
This analysis of the TWFET structure continues, as described in the U.S. Pat. Nos. 5,627,389 and 5,861,644, by use of an extension of Tripathi""s method to obtain an impedance or admittance matrix for the coupled transmission line structure. (V. K. Tripathi, xe2x80x9cAsymmetric Coupled Transmission Lines in an Inhomogeneous Medium,xe2x80x9d IEEE Trans. Microwave Theory Tech. vol. MTT-23(9), pp. 734-739 (September, 1975)) In the case of the TWFETs presented here, a 2-port impedance matrix is obtained for the coupled TWFET structure, due to the presence of two open circuit terminations which are located at opposite ends of the different coupled transmission lines of the dual gate TWFET. As noted in the patents referenced earlier, these two open circuit terminations allow a 2-port impedance matrix to be obtained from the TWFET coupled transmission line equations as a function of the coupling length, z0. This 2-port impedance matrix can be converted to a 2-port admittance matrix. Also, the 2-port quantities U (Mason""s U-function), MAG (Maximum Available Gain), MSG (Maximum Stable Gain), and k, the 2-port stability parameter can be calculated. Because the 2-port admittance matrix is calculated for a fixed frequency as a function of coupling length, these 2-port gain and stability parameters are also calculated at that frequency as a function of coupling length.
A traveling wave field-effect transistor operated at frequencies in the microwave range or above the microwave range, and having traveling wave signals propagating in a direction therethrough generally from and to electrodes attached thereto, comprising: semiconductor structure defining a traveling wave signal propagation direction and a transverse direction configured at right angles to said traveling wave signal propagation direction, said structure in cross section taken in said transverse direction perpendicular to said traveling wave signal propagation direction, said cross section corresponding to a cross section field-effect transistor,
a coupling length of said structure in said traveling wave signal propagation direction having electrodes configured for attaching transmission lines for an input signal and for an output signal, said input and output attachments at opposite ends of said coupling length,
at least one gate electrode extending along said coupling length in the traveling wave signal propagation direction,
at least one source electrode extending along said coupling length in the traveling wave signal propagation direction,
at least one drain electrode extending along said coupling length in the traveling wave signal propagation direction, wherein a traveling wave field-effect transistor is formed,
input transmission line attached to the electrodes at one end of said coupling length for an input signal, output transmission line attached to the electrodes at said opposite end of said coupling length for an output signal, a depletion region generally beneath said at least one gate electrode, said depletion region, when viewed in a cross section of said semiconductor structure taken in said transverse direction, having an edge,
means for positioning said edge between said at least one gate electrode and said at least one drain electrode region, and
means for separating the depletion region edge from the at least one drain electrode region,
said gate source and drain source electrode pairs having an electrode series resistance, and
in a plane transverse to said direction of signal propagation, there is the addition of cross section field effect transistor channel material, extending between the gate electrode and source electrode to increase the distance between the gate and source electrode which creates a gate-source lateral spacing region,
or,
there is the addition of cross section field effect transistor channel material, extending between the gate electrode and drain electrode to increase the distance between the gate and drain electrode which creates a gate-drain lateral spacing region,
or,
there are additions of cross section field effect transistor channel material, which create both said gate-source and gate-drain lateral spacing regions, in which, said gate-source lateral spacing region extends between the gate and source electrode including an end portion of the depletion region edge which lies between the gate electrode and the source electrode and a neutral region which extends from this end portion of the depletion edge to the source electrode;
such that
the length of said neutral region is defined as extending from the depletion region edge to the surface of the source electrode in the direction of the shortest distance between the gate electrode and the source electrode,
and
the lateral depletion region depth is defined as extending from the surface of the gate electrode to the depletion region edge in the direction of the shortest distance between the gate electrode and the source electrode, and, in which said gate-drain lateral spacing region extends between the gate and drain electrode including the end portion of the depletion region edge which lies between the gate electrode and the drain electrode, and a neutral region which extends from this end portion of the depletion edge to the drain electrode;
such that
the length of said neutral region is defined as extending from the depletion region edge to the surface of the source electrode in the direction of the shortest distance between the gate electrode and the source electrode,
and
the lateral depletion region depth is defined as extending from the surface of the gate electrode to the depletion region edge in the direction of the shortest distance between the gate electrode and the source electrode, and, in which said gate-drain lateral spacing region extends between the gate and drain electrode including the end portion of the depletion region edge which lies between the gate electrode and the drain electrode, and a neutral region which extends from this end portion of the depletion region edge to the drain electrode;
such that at some frequency,
at some coupling length of the traveling wave field-effect transistor, at some value of the electrode series resistance, the value of maximum stable gain is increased, by the addition of said cross section field effect transistor channel material.
A method of improving the performance of a traveling wave field-effect transistor comprising the steps of: forming a semiconductor structure defining a traveling wave signal propagation direction and a transverse direction configured at right angles to said traveling wave signal propagation direction, said structure in cross section taken in said transverse direction perpendicular to said traveling wave signal propagation direction corresponding to a cross section field-effect transistor,
defining a coupling length of said structure in said traveling wave signal propagation direction having electrodes configured for attaching transmission lines for an input signal and for an output signal, said input and output attachments at opposite ends of said coupling length,
forming at least one gate electrode extending along said coupling length in the traveling wave signal propagation direction,
forming at least one source electrode extending along said coupling length in the traveling wave signal propagation direction,
forming at least one drain electrode extending along said coupling length in the traveling wave signal propagation direction, wherein a traveling wave field-effect transistor is formed,
attaching transmission line to the electrodes at one end of said coupling length for an input signal, attaching transmission line to the electrodes at said opposite end of said coupling length for an output signal,
depleting a region generally beneath said at least one gate electrode, said depletion region, when viewed in a cross section of said semiconductor structure taken in said transverse direction, having an edge, positioning said edge between said at least one gate electrode and said at least one drain electrode region, and separating the depletion region edge from the at least one drain electrode region
with said gate source and drain source electrode pairs having an electrode series resistance,
and
in a plane transverse to said direction of signal propagation, adding cross section field effect transistor channel material, which extends between the gate electrode and source electrode to increase the distance between the gate and source electrode which creates a gate-source lateral spacing region,
or,
adding cross section field effect transistor channel material, which extends between the gate electrode and drain electrode to increase the distance between the gate and drain electrode which creates a gate-drain lateral spacing region,
or,
adding cross section field effect transistor channel material, to create both said gate-source and gate-drain lateral spacing regions, in which, said gate-source lateral spacing region extends between the gate and source electrode including the end portion of the depletion region edge which lies between the gate electrode and the source electrode and a neutral region which extends from this end portion of the depletion edge to the source electrode; defining a length of said neutral region as extending from the depletion region edge to the surface of the source electrode in the direction of the shortest distance between the gate electrode and the source electrode,
and
defining a lateral depletion region depth as extending from the surface of the gate electrode to the depletion region edge in the direction of the shortest distance between the gate electrode and the source electrode,
and, in which
said gate-drain lateral spacing region extends between the gate and drain electrode including the end portion of the depletion region edge which lies between the gate electrode and the drain electrode, and a neutral region which extends from this end portion of the depletion edge to the drain electrode;
and
defining the length of said neutral region as extending from the depletion region edge to the surface of the drain electrode in the direction of the shortest distance between the gate electrode and the drain electrode,
and
defining the lateral depletion region depth as extending from the surface of the gate electrode to the depletion region edge in the direction of the shortest distance between the gate electrode and the drain electrode,
such that at some frequency,
at some coupling length of the traveling wave field-effect transistor, at some value of the electrode series resistance, the value of maximum stable gain is increased, by the addition of said cross section field effect transistor channel material.